Why does it cost more to print with 2 or 3 colours?
Because letterpress is not like your ink-jet printer, which can print multiple colours at once. 99% of letterpress machines are single colour each colour must be printed one colour at a time.
Each colour also requires its own plate be made. It also requires that the press and rollers be cleaned and prepared for the new ink and new plate. The new plate is placed in the press and positioned by hand to receive the second colour in perfect position and with the perfect amount of impression.
The sheets are then passed through the press to lay down the second colour. Due to the time and labour intensive nature of the letterpress process extra colours have additional costs involved.